1. Field of the Invention
This invention is generally directed to methods and apparatuses for achieving thermal protection of low temperature materials in a high heat flux environment, and, more particularly, to active cooling methods and apparatuses traditionally involving backside convective cooling or transpiration cooling.
2. Description of the Related Art
Many engineering applications involve long-term exposure of structural materials to high heat flux thermal environments. The use of lightweight or low cost materials in such environment is often times only possible if some sort of thermal protection system (TPS) is incorporated into the design. The use of high temperature insulation or other passive approaches can result in unacceptably large mass/volume penalties, major supportability concerns, or severe operational restrictions. Long-term protection requires some sort of active cooling system, such as backside convective cooling or transpiration cooling.
A backside convective cooling TPS includes a thermally conductive material that transfers heat to a flowing coolant. Thermal management of the energy absorbed by the coolant significantly increases the complexity of this TPS. The coolant must reject the absorbed thermal energy at a heat sink or else be expelled and replaced.
A transpiration cooling TPS is the most thermally efficient active cooling approach, but its implementation has been severely limited due to difficulties imposed by the structural characteristics of porous materials. A typical transpiration TPS has a plenum bounded by an outer wall made from a porous material. The porous material has a large convective surface area per unit volume, providing highly effective cooling of the material and ultimately good thermal protection to the underlying structure. However, the choice of porous material for a transpiration TPS poses a difficult design problem. The two most attractive possibilities, porous ceramics and porous sintered metals, each have significant drawbacks. Porous ceramics tend to be brittle and have reduced structural strength compared to metals. Porous sintered metals tend to be stronger but are also heavier than porous ceramics, and thus may impose an unacceptable weight penalty.
Additionally, the need for a separate plenum for both backside convective and transpiration thermal protection systems adds greatly to the cost, weight and complexity of these systems. The need for a plenum in these systems may also be completely incompatible with applications requiring material continuity throughout the TPS.